Methttod for producing metal conductors on a substrate

ABSTRACT

The invention relates to a method for producing metal conductors, for instance copper conductor patterns as electronic components on a substrate, such as paper. Said method is particularly suitable for producing metal conductors on papers for large scale mass production using printing or like machines. In the method, an electroless deposition is carried out in at least two steps wherein a solution is made of one of the metallic starting material or the reducing agent, or the other one is present in a gas or vapour form, followed by successive application thereof on the substrate.

FIELD OF THE INVENTION

The present invention relates to a method for producing metalconductors, for instance copper conductor patterns as electroniccomponents on a substrate, such as paper. Said method is particularlysuitable for producing metal conductors on paper for large scale massproduction using printing or like machines.

PRIOR ART

In microelectronic industry, semiconducting devices that areincreasingly smaller in size and faster, are continuously developed.Metals, typically aluminium but also more recently, due to lowresistances thereof, increasingly higher amounts of copper and minoramounts of silver are used for the production of integrated circuits andmicrochips. Copper is also endowed with other desirable properties,including high thermal stability and low price. Production of copperfilms and patterns on various substrates is, however, often accompaniedwith problems, and the methods used are complicated. At present, mainlyUV photolithographic methods are used for producing copper film patternsat scales below 200 μm.

Direct printing of a copper pattern on a substrate using an ink-jetprinting method has recently been studied intensively due to thefollowing advantages:

-   -   the method may be carried out with a simple, inexpensive device        that may be controlled readily,    -   said printing method is safe and has no drawbacks,    -   printing is carried out directly without etching or complicated        surface treatments,    -   only low amounts of reagents are needed for printing, and the        energy consumption thereof being low.

The U.S. Pat. NO. 5,132,248 discloses the use of a colloidal coppersuspension for ink-jet printing method, followed by treatment atelevated temperature or laser treatment, and removal of excessivematerial.

The use of various copper precursors including organic copper compoundsis proposed for ink-jet printing method. After printing, however,heating at elevated temperatures must be carried out, the organic moietyof the compound being thus evaporated in the environment. Examplesinclude copper hexenoate described in Hong, C. M., Wagner, S., IEEEElectron Device Lett. 2000, 21, 384.

Metal films may be produced on substrates with the so-called electrolessdeposition method. Electroless deposition is defined as the controlledautocatalytic formation of a continuous film on a catalytic boundary dueto a reaction of a metal salt with a reducing chemical, in a solution.The reaction is normally carried out at the temperature of 30-80° C.,and no external power source is required for the reaction. The metal ionand reducing agent are present in the same solution, and, they react atthe catalytic boundary, or seed surface, typically comprising palladiumor tin. Suitable metals for said electroless deposition are nickel,copper, gold, palladium and silver. In this method, the metal uniformlycovers the surface to be treated and also penetrates into cavities andpores, but, however, the method is slow. Complexing agents are used forstabilizing the solution, but said agents also decrease the rate of thereaction.

The U.S. Pat. No. 5,158,604 describes a viscous aqueous solutionsuitable for electroless deposition, comprising metal ions e.g. copperor nickel, metal complexing agents e.g. EDTA, metal reducing agent e.g.formaldehyde or hypophosphite, and thickening agent e.g. xanthan gum,silica, or carboxymethylcellulose. The solution is applied on a heatedcatalytic substrate comprising metal or polymer, said substrate beingstationary or in form of a moving web, the solution being alsopreferably preheated before application thereof.

Document WO 00/33625 discloses a method for forming a conductive layeron a polymer substrate wherein ink containing catalytic particulatesilver, copper, etc. is printed on a substrate with a lithographicprinting method, followed by the immersion of said substrate into aconventional bath for electroless deposition, for providing a conductivelayer.

Electroless deposition is a known solution phase method for depositingmetal films on catalytic surfaces. As a process, said electrolessdeposition is too slow, and thus unsuitable for large scale massproduction. The reason for this is the fact that increasing ofconcentrations of the starting compounds in the solution would causeinstability of the solution and accodingly, homogeneous reactions wouldtake place. Moreover, the initiation of the deposition of the metal onthe substrate requires activation of the substrate surface, which istypically achieved with platinum. Known substractive lithographicprocesses of the prior art, wherein the desired pattern is etched, arenot suitable for mass production. In addition, the methods of the priorart are often expensive, and produce high amounts of wastes.

Accordingly, it is clear that there is an evident need for a method forproducing metal conductors, particularly metal conductor patterns onsubstrates, which method is especially suitable for large scale massproduction, and may be carried out with a printing machine or a similarapparatus at high speed. Moreover, the method should be simple, fast andinexpensive.

OBJECT OF THE INVENTION

The object of the invention is to provide a method for producing a metalconductor on a substrate.

Another object of the invention is to provide a method for producingmetal conductors, e.g. copper conductor patterns as electroniccomponents on a substrate.

Still another object of the invention is to provide a methodparticularly suitable for producing metal conductors on paper for largescale mass production using a printing machine or like apparatus.

Characteristic features of the method of the invention are presented inthe claims.

SUMMARY OF THE INVENTION

Now it has been surprisingly found that the problems and disadvantagesassociated with the prior art solutions may be eliminated or at leastsubstantially reduced by the method of the invention. In said method,electroless deposition is carried out in at least two steps. Metallicstarting material and the reducing agent are incorporated in separatesolutions, or one of them is present in gas or vapour form, saidsolutions or gasses or vapours being then successively sprayed orapplied on the substrate to sites where a film is desired.

DETAILED DESCRIPTION OF THE INVENTION

In the method according to the invention, electroless deposition iscarried out in at least two steps. In said electroless deposition, asolution is formed from at least one of the metallic starting materialand reducing agent, or one of them is present as gas or vapour, and thenthey are succesively applied on a substrate. Thus, separate solutionsare always made from the metallic starting material and reducing agent,or one of them is present as gas or vapour, said solutions or gases orvapours being successively sprayed or applied on the substrate to siteswhere a film is desired. As opposed to conventional electrolessdepositions, starting materials are incorporated in separate solutionsor one of them is present as gas or vapour, and therefore the growth ofthe metal film may be accelerated by increasing the concentrations ofthe starting materials, without simultaneously causing undesirablehomogeneous reactions. In the method according to the invention, atleast one of the starting materials is present in a solution, which issprayed on or the paper or other substrate to sites where a metal filmis desired. Said solutions are preferably aqueous solutions, but theymay, however, also comprise organic solvents such as alcohols.

Metals suitable for the method are selected from the group consisting ofCr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn,Sb, Te, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, and alloys thereof. Copper,silver, gold, chromium, iron, cobalt, nickel, palladium and platinum,and the alloys thereof are preferable. Particularly preferable arecopper, silver and nickel, in which high conductivity combines withfavourable price. The metal is introduced into the aqueous solutionsuitably as a salt, preferably as a sulphate or chloride. Said metalsolution contains said metal salt in a concentration varying between0.005 M and the concentration corresponding to a saturated solution,preferably from 0.1 to 0.5 M. Said metal solution is preferably anaqueous solution.

Said metal solution also optionally contains one or more complexingcompounds preferably selected from the group consisting of EDTA, citricacid, ethylenediamine.

The relative amount of the complexing compound is at leaststoichiometric with respect to the metal.

The pH of the metal solution is adjusted if necessary, the suitable pHrange depending on the metal used. EDTA complex of copper may bementioned as an example, for which the lower pH limit is 6, thepreferable range being from 12 to 13. Any suitable base, preferablysodium hydroxide, may be used for pH adjustment.

Suitable reducing agents include alkali and alkaline earth metalborohydrides, e.g. NaBH₄ and hypophosphites such as NaH₂PO₂,formaldehyde NCON, hydrazinhydrate N₂H₄, and aminoboranes R₂NHBH₃, wherethe group R may be an alkyl group, preferably a methyl, ethyl or apropyl group. The reducing agent is preferably used as an aqueoussolution.

Moreover in the solutions containing the metal and the reducing agent,surface active agents and agents controlling the surface tension may beused, if necessary, polyethylene glycol and sodium lauryl sulphate beingmentioned as examples.

The substrate is stationary, or it is a moving web, and futher, it maycomprise paper, board, other fibrous material, polymeric material, ormetal coated with a polymer. It is not necessary to catalyticallyactivate the substrate before application.

The number of the starting material solutions, gasses and vapours may bemore than one.

In the first step, one of the solutions of the starting materials, thatis, either the metal solution or the solution of the reducing agent, isintroduced onto the substrate surface using a suitable applicationmethod, suitably with conventional printing methods such as gravure,flexo, offset, silk screen, or ink-jet printing method, and preferablywith ink-jet printing method to the sites where a pattern is desirablyformed, or optionally on the whole surface. In the second step, theother starting material, that is the metal or the reducing agent, isthereafter brought on the surface of the substrate in form of a solutionusing a suitable application method, suitably with conventional printingmethods such as gravure, flexo, offset, silk screen, or ink-jet printingmethod, and preferably with ink-jet printing, thus either injected toform a pattern, or to cover the whole surface, or optionally vaporizedor as a gas. It is particularly preferable to use a digitally controlledink-jet printing method. The order of application of the startingmaterials is immaterial, and the application of the starting materialson the substrate may respectively be repeated several times.

Application may be carried out on the substrate using a suitableroll-to-roll printing method or on sheets, and further, the substratemay comprise paper, board, other fibrous material, polymeric material,or metal coated with a polymer. A roll-to-roll printing method ispreferably used.

The application is performed at a temperature depending on the process.For instance in copper process, the temperature is from 20 to 200° C.,preferably from 20 to 140° C.

The method of the invention has several advantages. The electrolessdeposition used to form the pattern may be carried out by applyingeither, or both of the starting components preferably in the form ofrespective solutions only to those sites where the metal deposition isdesired. The reaction of method according to the invention is fast sinceno stabilizers are needed. With this method, an electrically conductingpattern having a desired form may be produced using an additive methodon the substrate to the desired site, and the thickness of the patternmay vary over a wide range. The method may be performed at roomtemperature, at a normal atmosphere without any protective gasses. Thesolutions are aqueous and stable at room temperature, and moreover, thestarting materials are inexpensive. No waste is produced in the method,as opposed to the etching methods of prior art.

The invention is now illustrated by means of the following examples,without wishing to limit the scope thereof in any way.

EXAMPLES Example 1 Deposition of Copper on Paper

In the example, a solution of copper sulphate complexed withethylenediamine-tetracetic acid (EDTA) (0.25 M CuSO₄×5H₂O+0.25 M EDTA)was used as the starting copper material, and sodium borohydride (2,0 MNaBH₄) acted as the reducing agent. The pH of the copper solution wasadjusted to basic (pH 12-13) with sodium hydroxide (NaOH) before use.The copper starting material solution and the solution of the reducingagent were applied alternately on the paper at 140° C. in the air. Thecopper solution was allowed to spread on the paper for about 20 seconds,followed by the addition of the solution of the reducing agent. Thepaper was kept at 140° C. for about 2 minutes. As a result, a conductive(about 4-20Ω) copper layer was obtained on a filter paper (Whatman) byrespectively applying the two solutions three times, in amounts of 100μl, respectively.

The overall reaction:2 Cu(EDTA)²⁻+BH₄ ⁻+4 OH⁻→2 Cu+2H₂+B(OH)₄ ⁻+2 EDTA⁴⁻

Example 2 Deposition of Silver on Paper

In this example, a solution of silver nitrate complexed with ammonia(NH₃) was used as the silver starting material (0.04 M AgNO₃+0.01 NH₃),sodium borohydride (2,0 M NaBH₄) acting as the reducing agent. The pH ofthe silver solution was 12-13 before use. The silver solution andreducing agent were alternately applied on the paper, at 160° C. in theair. The silver solution was allowed to spread on the paper for about 20seconds, followed by the addition of the solution of the reducing agent.The paper was kept at 160° C. for about 2 minutes. As a result, aconductive (about 1-10Ω) silver layer was obtained on a filter paper(Whatman) by using a 100 μl application.

The overall reaction:8[Ag(NH₃)₂]⁺+BH₄ ⁻+10 OH⁻→8 Ag+BO₃ ⁻+16 NH₃+7 H₂O

1. Method for producing metal conductors on a substrate, characterizedin that an electroless deposition is carried out in said method at leastin two steps, wherein a solution is made at least from a metallicstarting material or from a reducing agent, or the other one being ingas or a vapour form, followed by the successive application thereof onthe substrate.
 2. Method according to claim 1, characterized in that themetal of the metal starting material is selected from the groupconsisting of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Tc, Ru, Rh, Pd,Ag, Cd, In, Sn, Sb, Te, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, and alloysthereof.
 3. Method according to claim 1 or 2, characterized in that themetal is copper, silver, gold, chromium, iron, cobalt, nickel, palladiumand platinum, and the alloys thereof.
 4. Method according to any ofclaims 1-3, characterized in that the metal is copper, silver or nickel.5. Method according to any of claims 1-4, characterized in that themetal is incorporated into the metal solution in the form of a saltsoluble in water, preferably as a sulphate or chloride, said metalsolution containing the metal salt in a concentration varying between0.005 M and the concentration corresponding to a saturated solution,preferably from 0.1 to 0.5 M.
 6. Method according to any of claims 1-5,characterized in that said metal solution also contains one ore morecomplexing compounds, preferably EDTA, citric acid, or ethylene diamine.7. Method according to any of claims 1-6, characterized in that saidreducing agent is an alkali metal or alkaline earth metal borohydride orhypophosphite, formaldehyde, hydrazinhydrate, or aminoborane R₂NHBH₃,where the group R represents an alkyl group,
 8. Method according to anyof claims 1-7, characterized in that said reducing agent is sodiumborohydride, formaldehyde, sodium hypophosphite, hydrazinhydrate, oraminoborane R₂NHBH₃, where the group R represents a methyl, ethyl or apropyl group, said reducing agent being preferably as an aqueoussolution.
 9. Method according to any of claims 1-8, characterized inthat the substrate is stationary or a moving web comprising paper,board, other fibrous material, polymeric material, or metal coated witha polymer.
 10. Method according to any of claims 1-9, characterized inthat one of the starting materials is applied as a solution on thesurface of the substrate using a printing method to sites where apattern is desirably formed, or optionally on the whole surface, andwhen the other starting material is applied as a solution on the surfaceof the substrate using a printing method, either to form a pattern byinjection, or sprayed to cover the whole surface.
 11. Method accordingto claim 10, characterized in that the printing method is a gravure,flexo, offset, silk screen, or ink-jet printing method.
 12. Methodaccording to claim 10 or 11, characterized in that said printing methodis a ink-jet printing method.
 13. Method according to any of the claims10-12, characterized in that said printing method is a roll-to-rollprinting method.
 14. Method according to any of the claims 10-13,characterized in that said printing method is a digitally controlledink-jet printing method.